BACKGROUND: With use of computed tomography (CT), intravenous contrast media is used routinely to help define anatomy and identify any pathology. Non-ionic iodinated contrast agents have largely replaced ionic agents and although rare, reactions to contrast are still important and more so with the continual increase in CT examinations in the last decade.OBJECTIVE: To examine the incidence, severity and risk factors of immediate hypersensitivity reactions to intravenous non-ionic iodinated contrast in CT.METHODS: Data of consecutive patients in an Australian tertiary hospital who developed immediate hypersensitivity reactions to intravenous iopromide during CT were collected and compared with the results of all contrast CTs performed over a four year period. Chi-square statistics and odds ratio are calculated on the variables of age, gender, referral source and seasons of the study.RESULTS: Forty-seven patients had immediate hypersensitivity reactions of 29,962 patients who underwent contrast CT (0.16%). Thirty-three patients (70%) had a mild reaction, 11 (23%) moderate and three (7%) severe. Sixteen (34%) were male and 31 (66%) were female. Sixty-eight percent were under 55-years of age. Reactions occurred in 0.35% (34 patients) of all outpatients, 0.07% (6 patients) of all emergency patients, and 0.06% (7 patients) of all in-patients. Eighteen (38%) occurred in spring, seven (15%) in summer, 17 (36%) in autumn and five (11%) in winter. There is a statistically significant higher risk of contrast reactions in females (Odds Ratio [OR] 2.41 p = 0.005), patients younger than 55-years old (OR 2.46, p = 0.005), outpatients (OR 5.42, p < 0.001) and CTs performed in spring and autumn (OR 2.77, p = 0.002).CONCLUSION: The incidence of immediate hypersensitivity reactions in contrast CT is low and mostly mild. Risk factors include female, younger than 55-years of age, outpatients and CT examinations performed in spring and autumn. This is the first study to observe such a seasonal variation.